Data whizzes around the world in packets, which each have a source, destination, a bunch of data and a method to determine where it belongs in a stream.
Problem is, they go all over the place. Literally: a stream of packets may all take a different route to the same destination. Most arrive, but some don’t. For that, we have TCP.
Transmission Control Protocol ensures that packets are reliably delivered and in order. Without it, who knows what the internet would look like.